The present invention relates to a system and method for reducing pollutants from the combustion of hydrocarbon fuel and, more particularly, to a system and method for recirculating flue gas in a controlled, optimimized manner to minimize NOx formation as a product of hydrocarbon combustion.
NOx is a common designation representing two oxides of nitrogen, nitric oxide (NO) and nitrogen dioxide (NO.sub.2). Together, these compounds react with hydrocarbons in the presence of oxygen and sunlight to form photochemical smog. It is for this reason that environmental concerns and attendant regulatory controls have required efforts to limit the amount of NOx generated by the combustion of hydrocarbon fuels.
Hydrocarbon-fired steam generators used for enhanced oil recovery are illustrative of this need and provide the preferred embodiment discussed hereinafter. In such applications, multiple furnace units and attendant steam generators are widely separated over an oil-bearing formation and must use available hydrocarbon fuel to convert water to steam for steam-flooding the underground formation. The feedstock fuel available is most often unprocessed or minimally processed natural gas or crude oil. Many different compounds may be present and mixed in such fuel, but a typical natural gas mixture might include:
______________________________________ Component Volume % ______________________________________ CH.sub.4 92% C.sub.2 H.sub.6 3% C.sub.3 H.sub.8 1% C.sub.4 H.sub.10 1% Other Hydrocarbons 3% ______________________________________
Typical combustion in a furnace unit for an enhanced oil recovery steam generator would yield combustion products as follows: EQU C.sub.A H.sub.B +O.sub.2 +N.sub.2 .fwdarw.CO.sub.2 +H.sub.2 O+N.sub.2 +NOx+CO
More specifically to point, the particular mechanism, thermal NOx production, responsible for oxidizing nitrogen in the ambient combustion air can be summarized as follows: ##STR1##
The elevated temperature within the furnace supplies the energy for oxygen molecules to dissociate and, as the temperature rises into the range of 2,800.degree. to 3,000.degree. F., the oxygen free radicals have sufficient energy to split bonds within the nitrogen molecules supplied by the combustion air. One of these nitrogen atoms combines with the oxygen and the other is sufficiently reactive to break another oxygen-oxygen bond, thereby forming another NOx molecule and producing another oxygen free radical to further propagate NOx production.
Without pollution controls, such combustion might yield NOx in the range of 0.06 to 0.1 pounds per million Btu fired.
However, it is known that recycling a portion of the combustion products in the exhaust or flue gas dilutes the oxygen concentration presented in the combustion air available for the combustion reaction and can significantly reduce NOx production. A key mechanism in reducing the NOx concentration is the effect that this dilution has on the temperature of the flame within the furnace. Significantly increasing the amount of inert gas in the combustion air increases the amount of gas which must be heated, but does so without correspondingly increasing the amount of oxygen available for combustion. Thus, the heat load drawing on the combustion reaction is higher and the recycled flue gas serves to lower the temperature of the flame within the furnace. This in turn reduces the formation of NOx as a combustion product because the reactions necessary for NOx formation are not favored by the lower reaction temperatures.
However, as discussed above, the NOx reduction is a sensitive function of the temperature of the combustion reaction and is materially influenced within a relatively narrow range. Thermal NOx production increases nearly exponentially once the combustion temperature exceeds a critical temperature in the range of 2,800.degree. to 3,000.degree. F. and unmodified combustion materially exceeds this critical temperature while ideal flue gas recirculation produces combustion temperature slightly below this. Thus, too much oxygen and the reaction temperature, and thereby the NOx concentration within the combustion products, substantially increases. Conversely, insufficient oxygen produces incomplete combustion which increases the concentration of carbon monoxide and other undesirable pollutants and potentially destabilizes the combustion reaction.
The prior art teaches control of the flue gas recirculation on a volumetric basis, either directly metering the flow rate of the flue gas returned or by performing a material balance utilizing the temperature of the flue gas, ambient air, and blended combustion air along with a known capacity for the blower drawing the ambient air into the furnace unit. A damper or other manual or automatic control means in the recirculation lines is then set based upon the calculated volume of recirculated flue gas. This may be enhanced by directly metering the volume of flue gas returning through the recirculation line to correspond to the calculated flow rate.
However, the prior art methods of reducing NOx produced are an indirect approximation and are not responsive to the realities of dynamic operation. Variations in the ambient temperature, furnace temperature, fuel composition, load on the furnace, etc. all render the use of such approximation techniques a crude tool to estimate the appropriate rate of flue gas recirculation. Further, it is necessary that the setting be substantially conservatively oxygen-rich in order to accommodate variations and inaccuracies in estimates because running the furnace too oxygen-lean risks unsafe and unstable combustion. Thus, the conservative safety margins necessary to account for the variations discussed above must be accommodated in a system and process that are very sensitive to even small variations. This results in less than optimal performance and materially increases the level of NOx produced during combustion.
The prior art has also approached reducing the NOx concentration in combustion products by manually or automatically controlling the capacity of the blower as a function of the concentration of unconsumed oxygen appearing in the flue gas. While this does serve to decrease the absolute amount of oxygen presented in the combustion air, it does nothing to alter the thermal load by increasing the ratio of inert materials to oxygen in the combustion air presented. Again, the commercially achievable results have been limited.